Fluid flow control or throttle valves are well-known in the art as a means for regulating rates of fluid flow. One type of fluid flow control device is a so-called flapper valve in which a thin "flapper" or plate-like member is disposed inside a fluid passageway and centrally mounted on a rotatable shaft passing laterally through the interior of the passageway. The plane of the flapper can thus be oriented by rotating the shaft in a clockwise or counterclockwise direction. The flapper is precisely dimensioned so as to close and more or less seal the passageway to stop or at least substantially reduce fluid flow when the plane of the flapper is oriented substantially perpendicular to the longitudinal axis of the passageway. Alternatively, rotating the shaft and the flapper 90.degree. or so such that the plane of the flapper is substantially parallel to the longitudinal axis of the passageway results in opening the passageway so as to permit fluid flow. The simplicity and ease of operation of such flapper valves makes them particularly well suited to regulating fluid flow.
A number of important industrial chemicals exist in the liquid phase at or about normal room temperature and pressure, but transition to the vapor phase under normal atmospheric pressure at elevated temperatures up to about 250.degree. C. For many industrial applications, it is preferred to handle these chemicals in the vapor phase while, at the same time, minimizing excessive, unnecessary inputs of thermal energy. Striking this balance, however, presents special problems in the case of throttle valves for regulating the flow of these vapor-phase chemicals. Unless all wetted surfaces of the valve are maintained at temperatures above the liquid-vapor transition temperature of the chemical being regulated, there is a danger of condensation on a valve interior surface resulting in possible corrosion of the valve, contamination of the fluid stream, and pooling of liquid adversely affecting valve operation.
In the past, this problem has typically been addressed by keeping the regulated fluid at a temperature higher than what might otherwise be required or desired and/or by applying an external heat source to the valve body. The internal flapper element would, in turn, be indirectly heated by a combination of conduction, convection and radiation. Because the flapper element is being heated only indirectly in such constructions, it typically has a lower temperature than other surfaces of the valve interior and therefore presents a prime site for unwanted condensation. Excessive thermal energy is thus required to prevent condensation in such a system.
Greater efficiency and better valve operation could be realized by the direct application of heat to the flapper element. In the past, however, this has not been considered possible because the flapper element must be free to rotate inside the valve body about a relatively narrow support shaft in order to permit valve operation without unduly blocking the fluid passageway when the valve is in the "open" position. No means have heretofore been available for direct application of heat to the internal flapper assembly of a flapper valve because of the accessibility problems.
These and other problems with and limitations of the prior art fluid control valves are overcome with the heated flapper valves of this invention.